1 Introduction

In flooded rice crops, the Fe3+ ions from iron oxide minerals are reduced to Fe2+ in the anaerobic conditions. The Fe2+ is more soluble chemical specie and, consequently, can be absorbed by rice plants, being able to cause toxicity affecting rice yield potential [1, 2]. At the same time, rice production annually generates a huge quantity of husks that can be converted into ash after burning in furnaces or boilers of grain processing industries [3]. When the rice husk is burnt under uncontrolled conditions, much of the organic matter is removed. Lignin and cellulose are oxidized, and the resulting material is composed of primarily silica (up to 95%) and other components such as iron, aluminum, calcium and potassium oxides [4] in variable quantity according to rice crop and harvest conditions, soil characteristics and methods of burning [5]. In general, rice husk ash is a material of difficult degradation, with high specific surface area, porous structure and some metal retention capacity [6].

When the organic materials are burnt in a controlled environment (i.e., temperature and time with little or no oxidation–pyrolysis), the production of biochar occurs. Biochar generally has a higher carbon content compared to ash, high CEC, high porosity and complexing ability with metals. Although biochars are obtained from biomass composed predominantly of cellulose, lignin and hemicellulose [7], other factors, such as the rate of heating and temperature and time of burning, can affect their characteristics even though they originate from the same type of biomass.

Ashes and biochars have been evaluated in environmental studies regarding their adsorption capacity of polluting elements or substances. The use of biochars for organic and inorganic pollutants removal from aqueous solutions has proved to be efficient for a large group of substances, such as dyes, pesticides and heavy metals [8, 9]. Studies using rice husk ash as an adsorbent material in solutions were also performed, including lead and mercury adsorption in aqueous systems [10] and the stabilization of lead and zinc [11].

The interaction of biochar and ash with heavy metals occurs with the inorganic silanols (Si–OH) groups present in ash structure or with organic functional groups, such as hydroxyls (–OH) and carboxylic acids (–COOH), including the possible presence of other functional groups from mineral oxides [12]. Recent studies evaluating adsorption of numerous metals, such as Cr, Cu, Pb, Hg, Zn and As, by activated carbon from coal also showed efficiency; however, the proper destination of the coals with adsorbed metals becomes costly. Therefore, studies with adsorptive materials obtained locally and available on a large scale without prior preparation and subsequent disposal are necessary [13].

In this context, we hypothesized that rice husk ash and/or biochar could be used as alternatives for iron toxicity mitigation in flooded rice crops, provided they have appropriate chemical and physical characteristics regarding iron adsorption. The objectives of this study were: (i) to characterize rice husk ash and biochar in relation to parameters such as elemental composition, ash content, functional groups, neutralization capacity, surface area, crystallinity degree and active sites and (ii) to evaluate the iron adsorption capacity of rice husk ash and biochar.

2 Materials and methods

2.1 Obtaining rice husk ash and biochar

Both rice husk ashes (RHA1) and (RHA2) were collected from two different rice trade companies Primo Berleze & Cia and processing of J. Fighera & Cia rice in Santa Maria, RS, respectively. Rice husks are used in furnaces to generate heat for grain processing, without controlling firing conditions (temperature or time), but RHA1 was more time exposed to the furnace than RHA2. The ash is disposed on the ground in open air, and collections were randomly carried out in piles, being packed in plastic bags for transportation and subsequent analysis. There was no pretreatment or drying done on material.

A biochar (RHB) was prepared from rice husk (collected at a rice mill in Santa Maria, RS) under slow pyrolysis conditions. The husks were pre-air-dried and then pyrolyzed for 1 h at 500 °C in a muffle furnace (Jung brand, model 7549) with 10 °C min−1 increase in temperature.

2.2 Chemical and physical characterization of materials

Total carbon (C) and nitrogen (N) contents of ashes/biochar were determined using an elemental analyzer (Flash model EA-1112, Thermo Scientifics). For elemental composition, 200 mg of each material was burnt in a muffle at 500 °C for 8 h and digestion was carried out with nitric acid + hydrogen peroxide [14]. Calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn) contents were measured by an atomic absorption spectrometer (AAnalyst 200—PerkinElmer), except for phosphorus (P) by using a spectrophotometer (BEL model S05) [15].

The cation exchange capacity (CEC) was determined according to Silva [16], and the neutralization power (NP) was determined by the same procedure used for lime described by Brasil [17]; however, the solutions used had to be diluted in relation to the original methodology, given the low neutralization power of the materials. Surface acid groups (carboxylic acids, phenols and lactones) were estimated by return titration as described in Boehm [18]. The ash content was determined according to ASTM D3172-13 [19]. The specific surface area (SSA) was determined by the BET procedure in an automatic determiner (Quantachrome Instruments) at the Ceramic Materials Laboratory (LACER) of the Federal University of Rio Grande do Sul (UFRGS).

The biochar and ashes samples were characterized by X-ray diffraction (X-ray diffraction DXA, Advance Bruker), after milling in mortar and pestle, sieved in 45-micron mesh and powdered in the equipment. The analyses were performed at room temperature with a copper tube (Kα = 1.5418 Ǻ radiation), in a range of 2θ of 10°–70°, with resolution of 0.02° and 0.6 s count time. The samples were also ground in mortar and pestle, mixed with potassium bromide (KBr), pressed and characterized by Fourier transform infrared spectroscopy (FTIR) in a spectrophotometer (Spectro One model, Perkin-Elmer) to identify functional groups at the Department of Material Engineering of the Franciscan University (UFN).

The samples were also evaluated by X-ray photoelectron spectroscopy (XPS) for surface characterization (qualification and quantification of the functional group elements on the surface of the solid material), as well as determination of the content of Si and O in the samples at the Department of Inorganic Chemistry of the Federal University of Rio Grande do Sul (UFRGS). For the evaluation by XPS, previously ground samples were placed in a spectrophotometer (Omicron-SPHERA), using an Al Kα radiation source (1486.6 eV) with an application to the anode 225 W (15 kV, 15 mA). The pressure used during the analyses was between 10–8 and 10–9 mbar. The detection angle of the photoelectrons (Θ) relative to the sample surface was set at 53° for all measurements. All binding energies on the spectra are referenced to C1s at 284.8 eV.

2.3 Iron adsorption capacity

Iron adsorption isotherms were determined using 0.5 g aliquots of previously ground triplicate sample of biochar/ash which were added to snap-cap flasks containing 50 mL of hydrated iron penta sulfate solution (Fe2SO4.7H2O) of 0, 5, 10, 20, 40, 80 and 120 mg L−1 concentrations. After shaking on a horizontal shaker for 24 h at room temperature (23 °C), a 20 mL aliquot was filtered on cellulose membrane (0.2 μm) and the iron concentrations were determined by atomic absorption spectroscopy. The initial pH of the iron solution was measured and that was around 3.8. The value was fixed in this pH due to avoiding the formation of soluble iron hydroxides that occurs above pH 5.5. From the experimental data, the equations were adjusted according to the Langmuir model [20]. The main idea was to use a model that can give an estimation of a maximum iron adsorption capacity to compare the biochar and ashes, and we believe that it was appropriate using Langmuir equations in our study.

$$ Q = \frac{{K _{L} \times C_{\max } \times C_{{{\text{sol}}}} }}{{1 + K_{L} \times C_{{{\text{sol}}}} }} $$
(1)

where Q = amount of Fe2+ in equilibrium; Cmax = maximum sorption capacity of Fe2+; KL (Langmuir's constant) = constant related to the affinity of the biochar/ash for Fe2+; Csol = concentration of Fe2+ in the solution.

The parameter Cmax is related to the maximum adsorption capacity and KL is the ratio between the adsorption kinetic constant and kinetic desorption constant [21]. The separation factor (RL) was also calculated. This parameter is used to predict the nature of adsorption (spontaneity) and is defined according to the equation [22]:

$$ {\text{RL}} = \frac{1}{{1 + K_{L} .C_{\max } }}. $$
(2)

If RL > 1, isotherm is unfavorable; if RL = 1, isotherm is linear; and if 0 < RL < 1, isotherm is favorable.

A sample of the ashes and biochar used in the isotherm experiment was oven-dried (60 °C) up to constant weight for evaluation by XPS by the same procedure described above.

2.4 Statistical analysis

The values obtained in the nutrient analyses of the ashes and the biochar were submitted to analysis of variance (ANOVA), and when necessary, the means comparison was performed by the Tukey test (5% level of significance).

3 Results

3.1 Chemical and physical characterization of materials

The RHB presented higher C content, followed by RHA1 and RHA2 (Table 1). Virtually, no N was detected in ashes, while a low concentration still persisted in biochar. Unlike C and N, higher Si content was verified in both ashes. There was no significant difference in Mn, Mg, Ca, Cu, Fe, K and P contents among materials, which are in relatively low concentrations, except for P and K, which presented higher values compared to the others. The materials presented low neutralization power (NP), although different from each other (Table 1). A higher CEC value was obtained in the RHB compared to the ashes. The presence of carboxylic radicals (–COOH) in the materials was not detected. The phenolic groups (–OH) were higher in relation to the lactonic groups (–COOR) in biochar and ashes.

Table 1 Chemical and physical parameters of rice husk biochar (RHB) and ashes (RHA1 and RHA2)
Full size table

The functional groups present in biochar and ashes evaluated by FTIR are very similar to each other (Fig. 1). The transmittance peak between 3678 and 3272 cm−1 present in three materials indicates the presence of hydroxyl groups (–OH). These groups may be derived from phenols, alcohols, ethers and esters. In all materials, the peak in 1617 cm−1 shows the stretch related to aldehydes. Stretches occurring at 1092, 1,097 and 1,095 cm−1 (for RHB, RHA1 and RHA2, respectively) confirm the presence of silanols (Si–OH) and siloxanes (Si–O–Si–OH) groups. In addition, a specific peak (793 cm−1) for Si–H in these materials was also verified.

Fig. 1
figure 1

Transmittance spectrum obtained by Fourier transform infrared (FTIR) biochar (RHB) and ashes (RHA1 and RHA2) of rice husks

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The three materials showed silicate structures in several crystalline arrangements (cristobalite, quartz and morganite) in the XRD evaluation (Fig. 2). It was possible to verify the presence of four mineral crystalline phases in the biochars diffractogram (RHB), including one containing Ca (Fig. 2a). Besides the identification of the silicate phases, diffractogram from biochar has a diffuse baseline, with no predominant peaks, characteristic of large amounts of amorphous components in its composition. In RHA1, occurrence of silicate phases (quartz and cristobalite) was also verified, as well as the presence of Ca in its crystalline structure (gyrolineite). The RHA2 presented phases with the presence of silica (cristobalite) and K.

Fig. 2
figure 2

X-ray diffraction of a biochar (RHB) and ashes b RHA1 and c RHA2 of rice husks

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The photoelectric spectrum obtained by XPS evaluation of the biochar surface (RHB) and ashes (RHA1 and RHA2) evidenced presence of O, C, Si and Ca (Fig. 3). The spectral lines of O (O1s) for RHB showed binding energy equal to 533.9 eV, and this is related to the presence of C = O type bonds, which confirms previously obtained information in FTIR evaluation and indicates presence of surface groups in the biochar such as phenols, ethers and hydroxyls. Carboxylic groups were previously discarded by Boehm method (Table 1). For ashes RHA1 and RHA2, value found was 532.8 eV and can be related to the presence of O to Si in the SiO2 form.

Fig. 3
figure 3

Photoelectric spectrum (XPS) of rice husk biochar (RHB) and ashes (RHA1 and RHA2) surfaces

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The XPS spectral line of Ca (Ca2p) had binding energy of 347.8 eV for all materials, and this value is related to presence of Ca3(PO4)2 in the three materials (Fig. 3). The spectral line of Ca was also present as Ca3p and binding energy of 26.8 eV, a value that relates the presence of Ca oxide in the biochar. The spectral line of C (C1s), whose binding energy was 284.8 eV in all materials tested, indicates that this element also carries out single bonds (sp3-type hybridization), fact previously confirmed by FTIR (stretches of type CH aliphatic). The spectral line of Si occurred under two binding energies (Si2s and Si2p). The binding energy values for Si2s were around 155.4 eV in the three materials and refer to Si in inorganic form (SiO2).

The XPS evaluation was also performed on the materials after exposure to the Fe2+ ions in the adsorption isotherm (Fig. 4). The binding energy values of elements O, Ca, C, Si and Ca remained the same. By means of deconvolution, it was possible to verify in a more detailed way different binding energies that compose peak formed in the primary region for the element Fe (Fe2p) (Fig. 5). There are the presences of FeSi2, FeSi, and Fe3Si (707.2 eV, 707 eV and 706.8 eV, respectively), of Fe–FeSO4 (711–713 eV), and of the bindings of Fe2+ in the oxide form (709.1 eV, 709.8 eV and 710.6 eV).

Fig. 4
figure 4

Photoelectric spectrum (XPS) of the biochar surface (RHB) and ashes (RHA1 and RHA2) of rice husk after eight days of contact with solution containing Fe2+ ions

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Fig. 5
figure 5

Deconvolution of Fe2p primary region in XPS spectrum of a biochar (RHB) and ashes b RHA1 and c RHA2 of rice husk after eight days of contact with Fe2+ solution

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3.2 Iron adsorption capacity

The iron adsorption capacity was determined by isotherms (Fig. 6). The Langmuir constant (KL), which indicates the adsorption affinity of adsorbate by adsorbent, also presented close values between the materials (0.10 mg L−1, 0.10 mg L−1 and 0.14 mg L−1 for RHB, RHA1 and RHA2, respectively), indicating similar affinity of these materials for iron. For the separation factor (RL), values found for RHB (0.7) and RHA1 and RHA2 (0.8) denote the occurrence of favorable and spontaneous iron adsorption. The maximum adsorption capacities of RHB, RHA1 and RHA2 were 5.53, 7.22 and 6.74 mg Fe2+ g−1, respectively.

Fig. 6
figure 6

Iron adsorption isotherms in a biochar (RHB) and ashes b RHA1 and c RHA2 of rice husk adjusted by the Langmuir equation. Error bars represent the standard deviation

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4 Discussion

4.1 Chemical and physical characterization of materials

The differences regarding C content in RHB, RHA1 and RHA2 were expected because of the higher oxygenation during ash production, guaranteeing greater carbon oxidation conditions converting organic carbon to CO2. The higher P and K contents in the biochar and ashes compositions justify their fertilizer effects on soil and plant uptake [23, 24]. Regarding lower values of NP, the study of Islabão [3] also showed a very low NP (0.91%) for rice husk ashes and the application of them can cause a significant increase in the soil pH only at doses above 40 Mg ha−1. Although higher soil pH can decrease Fe2+ in soil solution during flooded rice crops, rice husk ashes cannot be considered a viable alternative to replacing limestone for raising soil pH due to the large amount required to be applied. Although it is not as common to reach CEC values as high as that obtained for biochars in this study (112.5 cmolc kg−1), studies with biochar from cane residues found values close to 120 cmolc kg−1 [25]. CEC can be very variable among different biochars, since it depends directly on temperature of production and the feedstock [26]. The lower CEC of ashes may be related to lower SSA, similar to those found in other studies [23]. However, SSA can be strongly affected by temperature or chemical activation using potassium hydroxide in desiccated coconut residue as agricultural waste materials [35, 36].

Although a greater number of phenolic groups in the biochar are expected due to pyrolysis condition, there was no expectation to be found in the ashes, especially RHA1, due to the lower amount of carbon. However, the Boehm method [18] is usually used in higher carbon materials, and because of this, the methodology may present limitations for ashes characterization. As the principle involves an indirect estimate, other inorganic functional groups (e.g., silicates) may influence the results.

The results found by FTIR are very similar to those found in studies evaluating ashes and biochars from rice husk [23, 24]. The presence of organic functional groups described, as well as presence of silanols and siloxanes in ash, is responsible for ability of these materials to adsorb metals [23]. It was possible to identify possible CH stretches of aliphatic (2,923 and 2,850 cm−1), stretches of –C = O and –C–OH from ketones and aldehydes (1,636 and 1,615 cm−1), stretches of oxygen groups typical of lignin (1,200 cm−1) and metals such as K and Ca (620 cm−1) [24, 27]. However, it is necessary to take into account that FTIR results have a qualitative evaluation, not expressing the amount of the functional groups in the biochar and ashes. Hence, the amount of these groups can directly affect iron adsorption capacity of these materials.

Most of the studies with XRD that have evaluated rice-derived biochars converge to the appearance of peaks related to SiO2 structures represented in various arrangements and with elements such as Ca and K [24]. The formation of crystalline phases in the biochar is usually related to obtaining this material at high temperatures (700 to 800 °C), but this was also substantiated in studies where this material was obtained at 500 °C [28]. Studies have shown that peaks of minerals with Ca increase at temperatures of up to 500 °C, but decrease and even disappear at higher temperatures (700–800 °C). In general, the appearance of crystalline structures containing Ca in biochars also depends on the biomass from which they were obtained. The study of Srivastava [23] also reported the presence of SiO2 in the form of cristobalite and compounds with the presence of K in crystalline phase of rice husk ash. Therefore, when comparing ash, it is possible to affirm that firing conditions of the first one (RHA1) were more propitious to the formation of crystalline structures in relation to the second ash (RHA2). However, RHA2 ash did not present peaks related to the presence of quartz in its diffractogram. It is important to point out that the visualization of amorphous structures by this technique is not possible, which does not exempt such structures from actively participating in ion adsorption reactions.

In both ashes, the XPS value around 25.3 eV refers to this element in the form of carbonate [29]. The values in Si2p were recorded only in RHB and RHA1 (104 and 102 eV, respectively) and confirmed the presence of Si in the form of SiO2 and amorphous silicon (SiCxOy) [30]. The clusters found by XPS for the rice bark derived biochar are in agreement with other studies [9], in addition to being very similar to those observed by FTIR. This reinforces the need for use of analytical techniques that complement each other in order to confirm functional groups are on surfaces of materials in characterization studies. The main difference observed in photoelectric spectrum of these materials after exposure to the Fe2+ ions in the adsorption isotherm was the appearance of Fe-related bonding energies in the Fe2p and Fe3p regions, evidencing the Fe2+ sorption of the solution in the biochar and the ashes tested. The less intense binding values (Fe3p) are related to the appearance of Fe in FeSO4.7H2O form [31].

By means of deconvolution of XPS, the spectral lines of Fe (Fe2p1/2 and Fe2p3/2) found in a value close to 710 eV denote the presence of this oxygen-bound element (Fe2O3) [32]. However, it is necessary to point out that such a structure can be adsorbed in both organic groups and in silicate structures, provided there is an electrically favorable site for this phenomenon to occur. The less intense line of Fe (Fe3p) also refers to the same structure (Fe2O3) found for the other peaks of the element [31]. In general, the results obtained in this study indicate potential adsorption capacity of iron by ash and biochar but involving different functional groups.

4.2 Iron adsorption capacity

The ashes have a similar adsorption isotherm to biochar (RHB). In general, three isotherms obtained can be classified as being type L (Langmuir—subgroup 2). According to classification of Giles [33], the isotherms have an initial downward curvature due to a decrease in availability of active sites. It also indicates that there was saturation of the surface at which adsorbate has more affinity for solvent than for the already adsorbed molecules. However, the adsorption of Fe to biochar and to ashes must have occurred by different functional groups and mechanisms that together ended up providing similar total capacity values, even with the biochar having the possibility of a greater retention. Studies of the iron adsorption by biochar derived from rice husks are scarce; however, when these materials are used for other metals adsorption, such as cadmium and nickel, it is possible to verify that the results are also significant [34].

Considering parameters evaluated in characterization of materials together with adsorption results evaluated in solution, it was possible to consider that rice husk biochar and ashes have different characteristics, but a similar Fe2+ adsorption capacity. Further studies are necessary to evaluate their addition in rice crops as an economically viable and ecologically correct alternative to minimize or even eliminate the effects of iron toxicity.

5 Conclusion

Biochar and ash of rice husk have different levels of C, Si, CEC, SSA and some characteristics of the main surface functional groups, which should be related to the difference in oxygenation and temperature during the pyrolysis of the materials. However, these differences did not result in a large difference in adsorption capacity of Fe2+ ions. The iron adsorption capacity of the biochar was 5.53 mgFe2+ g−1 and of the ashes was 6.74 and 7.22 mg Fe2+ g−1 for the two materials tested, which demonstrates potential of these materials to mitigate iron toxicity in flooded rice crops.